Anharmonic Effects on the Squeezing of Axion Perturbations

Author:

Valentina Danieli, Takeshi Kobayashi, Nicola Bartolo, Sabino Matarrese, Matteo Viel

Keyword:

High Energy Physics - Phenomenology, High Energy Physics - Phenomenology (hep-ph), Cosmology and Nongalactic Astrophysics (astro-ph.CO), General Relativity and Quantum Cosmology (gr-qc), High Energy Physics - Theory (hep-th), Quantum Physics (quant-ph)

journal:

date:

2023-09-21 16:00:00

Abstract

It is assumed in standard cosmology that the Universe underwent a period of inflation in its earliest phase, providing the seeds for structure formation through vacuum fluctuations of the inflaton scalar field. These fluctuations get stretched by the quasi-exponential expansion of the Universe and become squeezed. From an observational point of view, if we consider Gaussian states, the expectation value of physical quantities on a squeezed state is indistinguishable from a classical average of a stochastic distribution. This renders cosmological perturbations arising from quantum fluctuations of free fields effectively identical to those with a classical origin. The cosmological squeezing has been largely studied in the literature, however most works have focused on nearly free fields. The aim of this paper is to deepen the understanding of the quantum-to-classical transition considering the effect of self-interactions. For this purpose, we study axion-like fields. In particular we follow the evolution of the axion's fluctuation modes from the horizon exit during inflation to the radiation-dominated epoch. We compute Bogoliubov coefficients and squeezing parameters, which are linked to the axion particles number and isocurvature perturbation. We find that the quantum mechanical particle production and the squeezing of the perturbations are enhanced, if one accounts for anharmonic effects, i.e., the effect of higher order terms in the potential. This effect becomes particularly strong towards the hilltop of the potential.

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